Yield of excited CO molecules from dissociative recombination of HCO+ and HOC+ ions with electrons

Abstract

The authors have investigated CO band emissions arising from the dissociative recombination of HCO(+) and HOC(+) ions with thermal electrons in a flowing afterglow plasma. The quantitative analysis of the band intensities showed that HCO(+) recombination forms the long-lived CO(a (3)Pi) state with a yield of 0.23+/-0.12, while HOC(+) recombination favors formation of CO(a' (3)Sigma(+)) and CO(d (3)Delta) with a combined yield of greater than 0.4. The observed vibrational distribution for the CO(a) state reproduces theoretical predictions quite well. The vibrational distributions for CO(a') and CO(d) are, in part, inverted, presumably as a consequence of a change in CO equilibrium bond length during recombination. The observations are compatible with current knowledge of the potential surfaces of states of HCO and HCO(+).

The chemistry of protonated species in the martian ionosphere

“…Although the COða À XÞ transition is dipole forbidden, the CO(a) state has a relatively short radiative lifetime, which varies with vibrational level and rotational levels between about 1 ms and 100 ms. A recently measured lifetime for the (v ¼ 0; J ¼ 1) level is about 3.7 s, and the average value for the whole system is similar (e.g., Slanger and Black, 1971;Jongma et al, 1997;Gilijamse et al, 2007). These bands were observed in the DR of HCO + by Adams and Babcock (1994), and the yield of the excited state was determined to be about 0.23 for vibrationally relaxed HCO + (Rosati et al, 2007;cf., Johnsen et al, 2003). Yan et al (2000) have suggested that the Cameron Bands detected from the astrophysical object ''the Red Rectangle'' arise from DR of HCO + , but the region is deficient in CO 2 .…”

Role of the direct mechanism in dissociative recombination of HCO+and HOC+

“…Dissociative recombination of this ion has been studied extensively both experimentally and theoretically. The experimental studies [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] agree on a relatively large rate of 2 × 10 −7 cm 3 s −1 at room temperature. Recent measurements [21] of the cross section of DR of H 13 CO + found that the cross section at low collision energies falls off like E −1 and is followed by a structured high-energy peak centered around 10 eV.…”

“…Since then, it has been observed in interstellar environments in both dense [30,31] and diffuse [32] molecular clouds. In measurements on DR using the flowing afterglow technique [17] or a storage ring [22] both isomers HCO + and HOC + are believed to be present. However, so far there is no direct measurement nor theoretical computation of the cross section for DR of HOC + .…”

“…However, so far there is no direct measurement nor theoretical computation of the cross section for DR of HOC + . For the modeling of the chemistry of the afterglow plasma [17], it was assumed that the DR thermal rate coefficients of HCO + and HOC + are of the same magnitude. For the models of the interstellar chemistry it is assumed that the thermal DR rate coefficient of HOC + is 1.3 × 10 −7 cm 3 s −1 [18,32,33], that is, smaller than that of HCO + .…”

Dynamics of proton transfer from ArH+ to CO

“…The capture rate k c = 8 ×10 −10 cm 3 s −1 is smaller than the reported value but within two standard deviations of the experimental result. While recent models [18,19] assume HCO + as the only product, experimental or theoretical evidence for the product partitioning into HCO + and HOC + is still missing [26].…”